a. Field of the Invention
This invention pertains to the detection of patterns in strings of binary digital data. More particularly, this invention pertains to the detection of a pre-specified sequential pattern of bits in a serial stream of bits for the purpose of detecting the synchronization pattern of bits in a stream of data received by a ground station from a remote sensing satellite.
b. Description of the Prior Art
Remote sensing satellites typically transfer the content of their observations to ground stations by sending a serial string of binary data to the ground station. The satellites usually transmit the content of their observations in a pre-specified binary format and preface the data by a pre-specified sequential pattern of bits, which pattern serves as a synchronization marker that the ground station uses to synchronize its processing of the subsequent bits in the stream.
The classic method for detecting a pre-specified sequential pattern of m bits in a serial stream of bits is to feed the stream of bits, one bit at a time, into a shift register having m stages. A logic network, that is "hard-wired" to the shift register, is configured so as to output a "true" indication only when the pattern of m bits within the shift register matches the pre-specified sequential pattern. Although the classic method is simple in conception and fast in operation, the logic network has to be "hard-wired" for a particular pre-specified pattern and the "hard-wiring" has to be changed whenever the pre-specified pattern is changed.
An alternative method is to use a computer to examine the stream of bits, one bit at a time, and, by means of an algorithm stored within the computer, to "compute" whether or not the immediate string of m bits corresponds to the pre-specified sequential pattern of m bits. Because the algorithm can be changed easily, the method can be easily adapted to detect a new pre-specified sequential pattern. The time required for the execution of the algorithm by the computer, however, limits the speed of this detection method.